1. Field of the Invention
The present invention relates to a zoom lens driving apparatus for electrically controlling the zooming of a camera.
2. Description of the Prior Art
For zooming the zoom lens of video cameras and the like, generally, one method drives an image magnification varying lens section, which is a part of the zoom lens and has an image magnification varying function, by a motor via a power transmission mechanism. In this respect, the image magnification varying lens section is usually driven at a predetermined constant speed for simplifying the driving circuit.
Recently, as the focusing system of the zoom lens, a so-called inner focusing system has been put in practice which is arranged such that a second lens component or a more rear side lens is driven. This system, as compared with the front focusing system, is light in weight with respect to the focus adjusting lens section, small in driving force, and short in minimum shooting distance.
However, the zoom lens of the inner focusing system causes a focal shift of the lens in response to a zooming operation. In the zoom lens of the inner focusing system, the moving distance s of the focus adjusting lens section (composed of the second lens and a more rear side lens) along the optical axis of the camera relative to a position at which an infinite-point object is focused is given by the following approximate expression: EQU s.apprxeq.f.sup.2 /u (1)
where f is the focal length of the zoom lens and u is the camera-to-object distance.
FIG. 2 shows a relationship of the expression (1) between the focal length f of the zoom lens (abscissa) and the moving distance s of the focus adjusting lens section (ordinate). That is, s=O for u=.infin., and s=f.sup.2 /u.sub.near, for the minimum shooting distance (u=u.sub.near).
As is clear from expression (1), the moving distance s of the focus adjusting lens section is proportional to the square of the focal length f. Accordingly, in the zoom lens of the inner focusing system, even if the object distance u is constant, the moving distance (i.e. the focused position) of the focus adjusting lens section varies as the focal length varies. In FIG. 2, a point N corresponds to the focal length f=f.sub.N of the zoom lens, indicating that the lens is focused on an object positioned at the minimum shooting distance (the moving distance amount of the focus adjusting lens section =s.sub.N). If the zoom lens is subjected to zooming such that the focal length varies from f.sub.N to f.sub.W (f.sub.N &gt;f.sub.W) with the position of the object kept unchanged and the focus adjusting lens section also unchanged, the zoom lens assumes the state represented by a point W in FIG. 2 (the moving distance of the focus adjusting lens section =s.sub.N), resulting in a large focal shift (.vertline.s.sub.N -s.sub.W .vertline.) where s.sub.W =f.sub.W.sup.2 /u .sub.near.
Generally, there is no linear relationship between the position D of the image magnification varying lens section and the corresponding focal length f of the zoom lens. Taking a conventional four-component zoom lens for a video camera as an example, FIG. 3 shows an example of the relationship between the position D of the image magnification varying lens section of the conventional zoom lens and the corresponding focal length f of the zoom lens. In FIG. 3, the abscissa represents the position D of the image magnification varying lens section (the amount of shift of the second lens component) relative to the wide angle end, while the ordinate axis represents the corresponding focal length f of the zoom lens. In the conventional four-component zoom lens for the video camera, as shown in FIG. 3, the amount of change (df/dD) of the focal length f of the zoom lens with respect to a change of the position D of the image magnification varying lens section has a tendency to increase as the focal length becomes larger.
Assume that a zooming operation is performed with the object distance unchanged, and differentiating both sides of the expression (1) by time t, the following expression is obtained: EQU (ds/dt)=(2f/u).multidot.(df/dt).multidot. (2)
By modifying the expression (2), the following relationship is obtained: EQU (ds/dt)=(2f/u).multidot.(df/dD).multidot.(dD/dt).multidot. (3)
Assuming that the zooming operation is performed at a fixed speed, the image magnification varying lens section is driven at a predetermined constant speed, i.e., (dD/dt) in the expression (3) is constant, so that the temporal change (ds/dt) of the moving distance s of the focus adjusting lens section is proportional to (df/dD). Thus, as described above, (df/dD) increases as the focal length f becomes large, and the amount of the focal shift increases.
Nowadays, from the viewpoint of operability, an auto focus adjusting mechanism has been becoming indispensable to the camera with the zoom lens built therein. However, where the auto focus adjusting mechanism is incorporated with the zoom lens of the inner focusing system, as described above, the amount of change of the focal shift caused by zooming becomes large, especially on the large focal length side. Consequently, the response of the auto focus adjusting mechanism cannot follow such a change, resulting in a severe out-of-focus state or mis-operation.
On the other hand, a focus correcting device effective during zooming is publicly known which makes a calculation using the position information of the focus adjusting lens section and of the image magnification varying lens section of the zoom lens and controls the focusing lens section to correct an out-of-focus state caused during zooming (Japanese Laid-Open Patent Application No. 49-115322). However, also in this system, the amount of change of the focal shift due to zooming increases on the large focal length side, so that a correcting operation cannot follow the change, thereby degrading the lens response.